Recently, there has been growing interest in energy storage technologies. As energy storage technologies are extended to devices such as cellular phones, camcorders and notebook PC, and further to electric vehicles, demand for high energy density of batteries used as a source of power supply of such devices is increasing. Therefore, research and development of lithium secondary batteries, which most meet the demand, are actively being conducted.
Among secondary batteries currently used, a lithium secondary battery developed in the early 1990's comprises an anode made of carbon material capable of intercalating or disintercalating lithium ions, a cathode made of lithium-containing oxide, and a non-aqueous electrolyte solution obtained by dissolving a suitable amount of lithium salt in a mixed organic solvent, and is prepared by storing such components in a case.
Such a lithium secondary battery is conventionally prepared by inserting an electrode assembly having electrodes on both sides of a separator in a battery case, and introducing a non-aqueous electrolyte solution into the battery case.
However, in order to introduce the non-aqueous electrolyte solution in the battery case, the battery case needs to be cut before introduction and then be closed after introduction, which brings about a great amount of inconvenience and may also damage the materials of the battery case. Until now, there has been no efficient solution for such a problem.
Meanwhile, overheating of lithium secondary batteries may cause thermal runaway or a puncture of a separator may pose an increased risk of explosion. In particular, porous polyolefin substrates commonly used as separators of lithium secondary batteries undergo severe thermal shrinkage at a temperature of 100° C. or higher in view of their material characteristics and production processes including elongation. This thermal shrinkage behavior may cause a short circuit between a cathode and an anode.
Accordingly, various researches have been made in order to prevent such a short circuit between a cathode and an anode even though batteries malfunction.